Electric motors are nearly ubiquitous today and range from very small in size, such as those found in compact disk players, to very large such as those found in industrial applications. The type of motor, its size, power and control requirements all depend on the particular implementation.
In general, electric motors include a shaft, a rotor and a stator. Driving a workpiece may be as simple as connecting the shaft to a platen for spinning an object such as a compact disk in a compact disk player. Alternatively, the shaft may drive the workpiece through one or more gears or transmissions for imparting rotational force under desired torque conditions or for imparting translational force.
There are several different types of DC electric motors, including brush and brushless motors. In brush motors, the rotor includes coils called the armature which must be connected to a power source to create torque on the rotor. The connection of the rotor coils to the power source is made through brushes, typically carbon, which slide over a metal cylindrical surface that is part of the rotor. The stator includes either a fixed permanent magnet or fixed coils which exert torque on the rotor via the armature.
There are several problems associated with brush motors, most of which relate to application of power to the rotor through the brush itself. These problems include wear of the brush and rotor contact during use, arcing, resistance and heating at the brush-contact interface, and burning of the brush during temperature extremes.
In brushless motors, permanent magnets are implemented in the rotor instead of coils. The stator includes fixed coils that may be selectively energized to create torque on the rotor. Because the permanent magnets do not require connection to a power source, no brush is required. Thus, the problems associated with the brush-rotor contact interface are avoided. Brushless motors tend to be more reliable over time than brush motors and are ideal for aerospace applications.
All brushless electric motors must be commutated in order to create torque and rotation on the shaft. During commutation, one or more coils of the stator are momentarily energized in a rotating fashion around the axis of rotation of the rotor. Each energized coil creates a magnetic field which imparts electromotive force ("EMF") between the energized coil and a magnetic pole of the rotor. It is the selective energizing of the coils which imparts torque on the motor shaft.
Traditionally, commutation has been done electronically using electronic components. Electronic commutation is accomplished by using position sensors on the motor which determine the position of the rotor relative to the stator and a series of switches which energize the stator coils based on the rotor position. Electronic commutation is reliable but expensive and is difficult to implement when stator coil currents are high.
There is a need for a new commutation technique for brushless electric motors which does not require expensive electronics and which can handle high coil excitation currents. The technique needs to be inexpensive and reliable and should avoid problems associated with a brush-rotor contact interface.